824 REPORT—1885. 
I have not thought it necessary in the foregoing réswmé to discuss all 
the views concerning cleavage propagated prior to the development of 
the mechanical theory, since they were, for the most part, founded on 
very meagre evidence as to the facts. Mr. W. Hopkins,’ for instance, 
concluded fron mathematical considerations, that, if the direction of the 
cleavage-planes in a rock be determined by stresses to which it has been 
subjected, those planes ought to be either perpendicular to the direction of 
maximum normal pressure, or parallel to the planes of maximum tangen- 
tial stress, which make angles of 45° with the maximum normal pressure. 
From an examination of Mr. Sharpe’s distorted fossils only, he was led 
to fix on the latter supposition, whereas the truth of the former, which 
was that embraced by Mr. Sharpe himself, has since been amply con- 
firmed. 
Neither have I noticed explicitly the objections brought against a me- 
chanical explanation of slaty cleavage by Professors Sedgwick,? H. D. 
Rogers,? and others, for such objections are found to disappear on a fuller 
examination of both the theory and the facts, and are indeed implicitly 
answered by the more complete exhibition of that theory in its various 
applications. 
IV. The Mechanical Theory of Slaty Cleavage: the Direct Evidence of the 
Distortion of Cleaved Rocks. 
The most obvious information regarding the distortion which cleaved 
rocks have undergone is that derived from the contortions produced in 
intercalated beds of less yielding materials. A typical example in the 
cliffs near Ilfracombe was described and figured by Dr. Sorby,‘ and has 
been frequently copied.® Here a bed of coarse-grained sandy slate occurs 
among fine-grained shaley slate, and is seen to have been forced into a 
series of bold undulations, although the bedding at a short distance above 
and below is undisturbed. The axial planes of the undulations coincide 
with the cleavage-planes of the finer slate,® and the thickness of the sandy 
bed at the troughs and crests of the undulations is four times the thick- 
ness at the intermediate places. The clear interpretation of this section 
is that the whole has undergone a lateral compression of considerable 
amount, partly compensated by yielding in an upward direction perpen- 
dicular to the compression ; and further, that the direction of compression, 
as deduced from the position of the axial planes of the folds, is at right 
angles to the cleavage-planes. Similar phenomena may be observed in 
almost any of the slate-quarries of North Wales. 
Dr. Sorby also made use of the evidence of the greenish spots, 
apparently of concretionary origin, which are of common occurrence in 
the Welsh roofing-slates examined by him. These spots may be assumed 
1 «On the Internal Pressure to which Rock Masses may be subjected, and its 
Possible Influence on the Production of the Laminated Structure,’ Zrans. Camb. Phil. 
Soc., vol. viii. p. 456 (1847). 
* Synopsis of British Paleozoie Rocks, Introduction (1855). 
$ On the Laws of Structure of the More Disturbed Zones of the Earth’s Crust,’ 
Trans. Roy. Soc. Edinb., vol. xxi. p. 431 (1856). 
* Edinb. New Phil. Jowrn., vol. lv. p. 137 (1853). 
® Tyndall, Phil. Mag., 4th ser., vol. xii. p. 41 (1856). Phillips, Brit. Assoc. Rep., 
1856, p. 385. Liyell’s Student’s Elements, p. 594, 2nd ed. (1874). Forbes, Pop. Set. 
Rev., 1870, &e. 
° Cf. fig. Quart. Journ. Geol. Soc., vol. xxxv. p. 88 (1879), where unsymmetrical 
flexures are seen to follow the same law. 
